How can I efficiently find the "simplest" rational in an interval?

Question

For a hobby software project I am working with exact rational arithmetic, as it happens this produces numbers $\frac{n}{k}$ of huge size even after reducing them, I am searching for an efficient algorithm to "simplify" these rational with a specified error tolerance.

In my own working out the "simplicity function" I was trying to maximize was $S(\frac{n}{k}) = |\frac{1}{k}|$ because it seemed the simplest.

In the following I will focus on finding a rational with minimal denominator, but if you have a solution with a different definition of "simple rational" that still decreases $\log(n) + \log(k)$ it would still solve my problem.

So given 2 rational numbers $p,q \in \mathbb{Q}$ with $p\lt q$, and a $k \in \mathbb{N}^+$ we can find a rational $\frac{n}{k}$ s.t. $p \le \frac{n}{k} \le q$ iff $\lceil kp\rceil \le\lfloor kq\rfloor$ by choosing $n \in \left[\lceil kp\rceil,\lfloor kq\rfloor\right]\cap\mathbb{Z}$.

Also if $k \ge \frac{1}{q-p}$ then the set $\left[\lceil kp\rceil,\lfloor kq\rfloor\right]\cap\mathbb{Z}$ is never empty.

Is there a simply way to find the minimal $k$ such that $\lceil kp\rceil \le\lfloor kq\rfloor$?

In a sense this could be more mathematically formulated as finding a global maximum of a function like \begin{equation} f(n,k) = \left\{ \begin{array}{lll} \frac{1}{k} & \text{if} & p \le \frac{n}{k} \le q \\ 0 & \text{if} & \text{otherwise} \end{array} \right. \end{equation} Or

\begin{equation} f(n,k) = \left\{ \begin{array}{lll} \frac{1}{\log(|n|)+\log(|k|)} & \text{if} & p \le \frac{n}{k} \le q \\ 0 & \text{if} & \text{otherwise} \end{array} \right. \end{equation}

But since it came from an algorithmic context I preferred to keep it in a semi-algorithmic formulation.

Answer 1

Here's a version of what has been said in the comments that's reformulated into an explicit algorithm, first assuming that $p$ and $q$ are irrational to avoid the edge case:

• Lazily compute the continued fraction expansions of $p = [a_0; a_1, a_2, a_3,\ldots]$ and $q = [b_0; b_1, b_2, b_3,\ldots]$: stop at the first $i$ such that $a_i \neq b_i$ (namely $a_i < b_i$ if $i$ is even, and $a_i > b_i$ if $i$ is odd, all this is for $p<q$).

• The sought-after rational is then $[a_0; a_1, a_2, a_3,\ldots, a_{i-1},\min(a_i,b_i)+1]$.

Indeed, this is clear from the paths taken in the Stern-Brocot tree: the branch toward $p$ from the root proceeds by going $a_0$ steps to the right, then $a_1$ to the right, then $a_2$ to the left, and so on; and similarly for $q$ with $b_j$ instead of $a_j$. The properties of the Stern-Brocot tree make it clear that the sought-after rational is the node of first divergence between these two branches; now the continued fraction expansion of the rational node in question is obtained by tacking a final $1$ to the common sequence, so it's $[a_0; a_1, a_2, a_3,\ldots, a_{i-1},\min(a_i,b_i), 1]$ which is more standardly written as $[a_0; a_1, a_2, a_3,\ldots, a_{i-1},\min(a_i,b_i) + 1]$.

If $p$ and/or $q$ are rational the same procedure applies provided we take the continued fraction expansion of the form $[a_0;a_1,\ldots,a_r,1]$ and/or $[b_0;b_1,\ldots,b_s,1]$, because again this tells us that $p$ is reached in the Stern-Brocot tree by going first $a_0$ steps to the left, then $a_1$ to the right, and ending in $a_r$ steps in the final direction (and stop there):

• If there is $i\leq\min(r,s)$ such that $a_i\neq b_i$, this is exactly as above.

• If $s\geq r$ and $b_i = a_i$ for all $0\leq i\leq r$, then the sought-after rational is $p = [a_0;a_1,\ldots,a_r,1]$ itself (it is an ancestor of $q$ in the Stern-Brocot tree).

• If $r\geq s$ and $a_i = b_i$ for all $0\leq i\leq s$, then the sought-after rational is $q = [b_0;b_1,\ldots,b_r,1]$ itself (it is an ancestor of $p$ in the Stern-Brocot tree).

Example: Suppose I want to find the rational with smallest denominator between $1.728 = [1; 1, 2, 1, 2, 10, 1]$ and $1.729 = [1; 1, 2, 1, 2, 4, 2, 2, 1, 1, 1]$: the algorithm in question shows that it's $[1; 1, 2, 1, 2, 4, 1] = 102/59$; and of course, this rational is also the rational with smallest denominator between itself and any of the two numbers I started with, which exemplifies the edge cases.